Link Aggregation Protocol (LAG) is a mechanism used to combine multiple links and treat them as single link for layer 2 purposes. This is covered by IEEE 802.3ad and IEEE 802.1AX-2008.
A multicast packet is a multi-destination packet, as opposed to unicast packets which are intended for a single destination.
Referring to FIG. 1, a lookup engine 102 generates a Multicast Group ID (MGID) from the control information of an incoming multicast packet. This Multicast Group ID maps to a link bitmap 104 in a link expansion vector table 106, the link bitmap 104 indicating links the packet should be transmitted on to reach its destination. Based on the link bitmap 104, a copy of the packet is sent to the corresponding links.
Referring to FIG. 2, for a LAG port 202, packets are sent over only one “preselected” link in that LAG port 202. In order to avoid duplicate packets and packet out-of-order problems, packets cannot be sent over all three links in the exemplary LAG port 202. Utilizing only a single link, however, is inefficient usage of the LAG bandwidth, which is the primary reason for using a LAG in the first place.